57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The langasites have been a major focus of study in the last decade due to the variety of properties they exhibit including piezoelectricity in La₃Ga₅SiO₁₄ [1], highly degenerate magnetic ground states in Pr₃Ga₅SiO₁₄ [2], and multiferroicity in Ba₃NbFe₃Si₂O₁₄ [3]. The latter has been a particularly active area of research because of complex spin dynamics arising from chiral, helical antiferromagnetic ordering at T_N= 26 K. The chirality introduced by the magnetic structure breaks symmetry, which allows this material to undergo a ferroelectric polarization [4]. Efforts to increase the multiferroic transition temperature by chemical substitution by other research groups have been unsuccessful; the Fe³⁺ cation occupies a tetrahedral site that other magnetic ions find unfavourable compared to the octahedral Nb⁵⁺ site.

We have undertaken the task of synthesizing new multiferroic compounds based upon this structural motif.  Drawing on previous work from B.V. Mill [5], we have optimized a method to substitute Fe³⁺ for other magnetic cations by taking advantage of Te⁶⁺, which is only known to exist in octahedral coordination [6]. Te⁶⁺ has a high affinity for the Nb⁵⁺ site, forcing magnetic ions like Co²⁺, Mn²⁺, and Cu²⁺ into the tetrahedral site.  We have recently synthesized isostructural compounds Pb₃TeCo₃V₂O₁₄ and Pb₃TeCo₃P₂O₁₄, members of the dugganite family.  Both of these compounds have a series of magnetic transitions at low temperatures (near 8.0 K and 6.6 K) that are believed to be multiferroic in nature (that is, they involve both magnetic ordering and evidence for electric polarization).  These are the first compounds beyond the Ba series that have been characterized.  Although our efforts to increase the ordering temperature have failed, the underlying magnetic behavior of this system is unlike any other langasite studied thus far; the dugganite has two magnetic transition temperatures at T_N1 = 8.6 K and T_N2 = 6.0 K (Figure 1) more reminiscent of other Co‐V systems like Co₃V₂O₈ and monoclinic Co₂V₂O₇.  A remarkable reordering of Co²⁺ moments occurs within a 2.6 K range. The first magnetic structure has a propagation vector k = (0.752, 0, 1/2) while the second has a propagation vector k = (5/6, 5/6, 1/2).  While the second structure has been studied in detail, knowledge of the first structure remains rather limited, though recent work suggests this phase may contain a dynamic component – that is, there is only a partial ordering of the spins at low temperatures.  The temperature dependence of the dielectric constant was measured to determine if this sample displays magnetoelectric coupling (figure 2).  An anomaly can be seen at each magnetic transition temperature in both compounds. Thorough investigation of the structure yields two possible causes for these anomalies. Firstly, short V‐O distance along the c axis seen across all members of the dugganite series may introduce antiferroelectric ordering. An alternative explanation may be the dynamical nature of the first magnetic phase. Perhaps a preferred moment orientation exists in a dynamic row of Co²⁺ moments, which coexist with static ordered moments (figure 3). Further study is needed to confirm these findings.

We have also had recent success synthesizing new members of the langasite series that preserve the original langasite structure.  Using the same strategy of the dugganite compounds, we prepared K₃TeFe₃P₂O₁₄, which has a much smaller lattice that Ba₃NbFe₃Si₂O₁₄.  The transition temperature appears to be increased to 41 K in this material, due to the increased magnetic exchanged between the Fe³⁺ ions.  Further tests are necessary to determine if the transition has a multiferroic origin.

Future studies include extending the Pb compounds to introduce other magnetic ions (such as Mn²⁺), single crystal growth of all of these materials by flux methods, and future testing of the multiferroic samples.  This project has enabled the funding of 4 undergraduate students, with two of them completing undergraduate thesis projects as a direct result of this work.  In addition, some of this work has been presented at major conferences such as the CSC in Calgary, Alberta earlier this year.

References

[1]  H. Fritze, J. Electroceram. 26, 122 (2011).

[2]  L.L. Lumata, T. Besara, P.L. Kuhns, A.P. Reyes, H.D. Zhou, C.R. Wiebe, L. Balicas, Y.J. Jo, J.S. Brooks, Y. Takano, M.J. Case, Y. Qiu, J.R.D. Copley, J.S. Gardner, K.Y. Choi, N.S. Dalal and M.J.R. Hoch, Phys. Rev. B 81, 224416 (2010).

[3]  H.D. Zhou, L.L. Lumata, P.L Kuhns, A.P. Reyes, E.S. Choi, N.S. Dalal, J. Lu, Y.J. Jo, L. Balicas, J.S. Brooks and C.R.Wiebe, Chem. Mater. 21, 156 (2009).

[4]  C. Stock, L.C. Chapon, A. Schneidewind, Y. Su, P.G.Radaelli, D.F. McMorrow, A. Bombardi, N. Lee and S.W. Cheong, Phys. Rev. B 83, 104426 (2011).

[5]  B.V. Mill, Russ. J. Inorg. Chem. 54, 1205 (2009).

[6]  A.E. Lam, An X‐ray Crystallographic Study of the Crystal Structures of Four Tellurium Oxysalt Minerals: Dugganite, Choloaite, Rodalquilarite, and Graemite. Dissertation, University of British Columbia, 1998.

[7]  H.J. Silverstein, K. Cruz‐Kan, A.M. Hallas, H.D. Zhou, R.L. Donaberger, B.C. Hernden, M. Bieringer, E.S. Choi, J.M. Hwang, A.S. Wills and C.R. Wiebe, Chem. Mater. 24, 664 (2012).

Figure 1:  Neutron diffractogram contour plot showing two magnetic transition temperatures at 8.6 K and 6.0 K in Pb₃TeCo₃V₂O₁₄.  Diffuse scattering, indicative of short-ranged ordering, appear at high temperatures [7].

Figure 2:  The temperature dependence of the dielectric constant (black) shows anomalies at each of the transition temperatures.  The first derivative of the dielectric constant (red) shows these anomalies more clearly. [7]

Figure 3:  The magnetic structure of Pb3TeCo3V2O14 below 6 K shown from the (a) ab and (b) ac planes.  The magnitude of the moment changes over 6 unit cells (c).  An intermediate magnetic structure appears between 6.0 and 8.6 K that is illustrated in (d).  An example of a dynamic spin is circled in red. [7]